Magnetic heads are generally well known and are used for high resolution printing and other applications. For example, magnetic printing heads are disclosed in U.S. Pat. Nos. 4,291,314; 4,328,503 and 4,370,661. The patents generally disclose printed circuit boards with parallel conductors on the surface and a high permeability magnetic comb structure extending between the conductors to form a magnetic yoke around each conductor. These magnetic combs can be made by electrodeposition.
As disclosed, the printed circuits are flexible and generally positioned over subtrates which can be flat or slightly arcuate. However, these printing heads involve complicated structure and straight conductors which do not necessarily provide good resolution in the direction of the conductors.
It is also know from U.S. Pat. No. 4,097,871 to surround wires with a soft-magnetic material, the embodiment described therein has a very complex design, and its construction requires very great care. It moreover is difficult to achieve compaction of the magnetizing elements to achieve the resolution required in conventional printers. Finally, the conductive wires are substantially straight so that the magnetic field is poorly defined in a direction parallel to the direction of movement of the medium.
Another type of printing head is disclosed in UK Pat. No. 806,288. In this device, which is used in a magnetographic printer, parallel conductive wires are bent at regular intervals around a sheet of dielectric nonmagnetic material. The bent part is brought into contact with the magnetizable medium and then a strong current is fed for a brief time through selected wires to generate a magnetic field around said wires sufficient to locally magnetize the medium.
A disadvantage of this device is that a very high current strength is required to obtain a magnetic field strong enough to magnetize the medium. Extensive apparatus is required for switching the current on and off. If wires are positioned very close together as is customary in the printer art to obtain high resolution, there is a great risk that the wires will influence one another. Such influencing becomes visible when the magnetic image is subsequently converted to a visible image. Additionally, high currents cause heat to be developed in the wires which limits the duration and frequency of current pulses required to produce an image.
Another disadvantage of the device is that the magnetic field of the wires, and hence the magnetized area on the medium, is not sharply defined. Because the size of the magnetized area cannot be adequately controlled, the density of the magnetized areas that can be achieved in practice is low.
Accordingly, it is an object of the present invention to obviate the disadvantages of the type of printing array disclosed in UK Pat. No. 806,288.